The present invention relates to a transversal type smear-desmear filter for removing an intermittent noise and/or interruption caused by an impulse noise in a data transmission.
A smear-desmear filter is composed of a smear filter and a de-smear filter which have an opposite group delay characteristics with each other. When a smear filter is used in a transmit side, and a de-smear filter is used in a receive side, an intermittent noise energy is dispersed on a time axis, and then, the influence of that noise is removed without influence to a signal to be transmitted.
The effect of a smear-desmear filter to a transmission quality is described in the article by Richard A. Wainwright in IRE Trans. on COM. SYSTEMS, December, 1961, and the theory of that article is briefly described here.
FIG. 1 is a block diagram of a transmission system which utilizes a smear-desmear filter.
In FIG. 1, the reference numeral 1 is a transmitter, 2 is a smear filter, 3 is a de-smear filter, 4 is a receiver, 5 is an intermittent noise generated in a transmission line 10.
When a data signal at the output of the transmitter 1 has the frequency band width 0-f Hz, and that data signal is applied to the smear filter which has the delay characteristics as shown in FIG. 2(a), the data signal with the time duration t is prolonged to t+L in the smear filter, and that prolonged data signal is sent in the transmission line. That prolonged data signal can be restored to the original data signal by passing it through the de-smear filter 3 installed in a receive side having the opposite delay characteristics as shown in FIG. 2(b).
On the other hand, an impulse noise 5 superposed on a data signal in a transmission line passes only the de-smear filter 3, but does not pass the smear filter 2. Accordingly, a noise power is dispersed to the time duration L, and then, the influence of an impulse to a data signal becomes small. A smear-desmear filter having the linear characteristics of FIGS. 2(a) and 2(b) is described in the U.S. Pat. No. 4,285,045.
However, when a smear-desmear filter with the linear delay characteristics of FIGS. 2(a) and 2(b) is implemented by a transversal filter, which has a plurality of series connected delay line with weighted taps, and an adder for providing the sum of the outputs of the taps, the necessary number of the weighted taps increases rapidly with the increase of the necessary delay time. For instance, when a signal with the highest frequency 3.4 kHz is sampled by a sampling frequency of 6.8 kHz and the delay time of 10 msec is requested, the requested number of the weighted taps in the transversal filter is even 85. Further, when the necessary delay time is 34 msec, the number of the weighted taps in the transversal filter is even 259.
When the digital type transversal filter is used, each weighted tap causes a quantize noise since each tap provides a product of the level of a data signal and the tap weight. Therefore, it is preferable that the number of the weighted taps is as few as possible.